Experimentelle Untersuchung der prägenden Wirbelsysteme im Nachlauf stumpfer Körper und deren aktiver Beeinflussung

The flow around bluff bodies is dominated by different vortex structures, which are responsible for extensive momentum loss, low static base pressure and result in comparatively high pressure drag. In two-dimensional and quasi-two-dimensional bluff body flows, the main vortex mechanism is the periodic-alternating shedding of large-scale, transversal vortices, composing a stable vortex street downstream. Flow fields around three-dimensional bluff bodies are characterized mostly by stable, large-scale longitudinal vortex systems. Passive and active flow control approaches were applied to a blunt NACA0012 wing model in order to gain an insight into the mode of action of these flow control strategies using time-resolved PIV and base pressure measurements. Both the variation of the geometry of the model rear end, as well as the tangentially, periodically-blowing compressed air jets located at the base edges, result in a considerable attenuation of the regular vortex shedding process, which is connected with a drag reduction of up to 37 % and a fundamental change of the dominating vortex mechanism. An actuation using a frequency out of the natural vortex shedding wave band yields a significant amplification of the large-scale, periodic-alternating vortex shedding and a distinct drag rise. Three-dimensional bluff body flows were investigated experimentally in a stereo-PIV configuration using an adapted AHMED body half model with a slant angle of f = 25 degree angle. This bluff body flow is characterized by a steady longitudinal vortex, which is formed at the lateral edge of the model slant out of the shear layer. This vortex is continuously fed by the lateral shear layer and leads to a very low static pressure in the effective range compared to the regular surrounding recompression on the model slant. The active flow control system applied to the AHMED body half model, using two continuously-blowing compressed air jets with a sufficient momentum transfer, results in a cutoff of the vortex and the feeding, lateral shear layer and causes a significant displacement of the vortex in upward direction, away from the slant. This leads to a reduced influence of the longitudinal vortex on the static pressure field on the rear slant and an almost two-dimensional development of the recompression regarding its spanwise characteristics.